// 2. 具体中介者：聊天室
public class ChatRoom : IMediator
{
  private readonly List<Colleague> _participants = new();

  public void Register(Colleague colleague)
  {
    _participants.Add(colleague);
  }

  public void Send(string message, Colleague sender)
  {
    foreach (var participant in _participants)
    {
      // 不发给自己
      if (participant != sender)
        participant.Receive(message, sender);
    }
  }
}